(1) Field of the Invention
This invention relates to power amplifiers and, more particularly, to switching power amplifiers that can efficiently and linearly amplify a signal.
(2) Description of Related Art
Amplifiers are electronic devices that are used for increasing the power of a signal, and are generally categorized into various classes. Reference is made to the exemplary U.S. Patents that disclose various types of amplifiers: U.S. Pat. Nos. 6,563,377; 6,498,531; 6,429737; 6,356,151; 6,297,692; 6,282,747; 6,246,283; 6,229,388; 6,097,249; 6,091,292; 6,078,214; 6,072,361; 6,016,075; 5,982,231; 5,973,368; 5,963,086; 5,838,193; 5,805,020; 5,617,058; 5,160,8969; 5,014,016; 4,531,096; and 3,629,616.
In general, class A amplifiers produce a linearly amplified replica of an input signal, but are inefficient in terms of power usage (generating a great amount of heat) because the amplifying elements are always biased and conducting, even if there is no input. With class A amplifiers, 100% of the input signal cycle is used to actually switch on the amplifying devices.
The prior art FIG. 1 A is an exemplary illustration of a conventional class A amplifier 100 that amplify a differential input signal over the whole of the input cycle, having a differential input 106 with input signal terminals 102 and 104. With class A amplifier 100, the amplifying elements 122 and 124 (also constituting a buffer output stage) are biased by voltage sources 126 (+VCC) and 128 (−VCC) and the current sources 112 and 120. The biasing of the amplifier 100 is such that the amplifying elements are always conducting to some extent, and are operated over the most linear portion of their characteristic curve (known as transfer function or transconductance curve). In general, class A amplifiers are inefficient in terms of power usage because the amplifying element(s) are always biased and conducting, even if there is no input signal to be amplified.
As illustrated in the prior art FIG. 1A, a typical class A linear amplifier 100 with differential input 106 is comprised of a negative or inverted input terminal 102 coupled with a base of a first transistor 108, and a positive or non-inverted input terminal 104 coupled with a base of a second transistor 110. A current source generator 130, through a current source 112, biases the differential input 106. The emitters of the first transistor 108 and the second transistor 110 are coupled with the current source 112 for providing a precise, constant current level to the differential input transistors 108 and 110 for maintaining the differential signal between the two inputs. The collectors of the transistors 108 and 110 are coupled with respective secondary current sources 114 and 116, with the secondary current sources 114 and 116 coupled with one another through a feedback 118. The secondary current sources 114 and 116 with the feedback 118 accurately impose and maintain the differential input current passing through the transistors 108 and 110. As further illustrated, the current passing through the transistor 110 is supplied to the base of the second output-amplifying transistor 124, with a base of the first output-amplifying transistor 122 being supplied via a current source 120. With class A linear amplifiers with a differential input, the output signal 132 is a linear, amplified replica of the difference between the input signals. Therefore, constant current sources 112 and 120 are used to maintain a constant output that accurately reflects the amplified differential input. Biasing of the output amplifying elements 122 and 124 by the voltage sources 126 (+VCC) and 128 (−VCC) and the current sources 112 and 120 prevents crossover distortions because the amplifying elements are always ON, generating the linearly amplified replica of the original signal (differential signal). However, this is also the cause for the inefficiency of this type of amplifier.
The push-pull class B amplifiers amplify a signal through the balance of non-saturated sink and source (push-pull) output stage sections. This arrangement provides excellent efficiency (compared to class A amplifiers) because there is no biasing of the output amplifying elements by current sources. That is, unlike the class A amplifiers, the amplifying elements of class B amplifiers are not constantly ON. However, this introduces crossover distortion caused by a small glitch 160 (FIG. 1B) at the “link” between the two halves of the signal generated by the sink and the source. Regrettably, most solutions to reduce the crossover distortion (the small glitch 160 at the link between the two halves of the signal) reduce the efficiency of the class B amplifiers.
The prior art FIG. 1B is an exemplary illustration of a class B linear power amplifier 140, which is comprised of an amplifying stage 142 and power stage 144. The amplifying stage 142 is comprised of an input terminal 146 that is coupled with a base of a first NPN Bipolar Junction Transistor (BJT) 148 and a base of a first PNP BJT 150. The emitters of the BJTs 148 and 150 are coupled together with ground GND. When a signal greater than the biasing signal of either transistor is applied to the transistors (assuming during a first half cycle of the input signal with a first polarity), the transistor 148 turns ON (+VBE), and the transistor 150 remain OFF. Activation of the transistor 148 places a first polarity voltage across the resistor 152 coupled in between the positive voltage source +VCC 154 and the collector of the NPN BJT transistor 148. The current generated due to the voltage across the resistor 152 passes through the collector-emitter junction of the NPN BJT transistor 148 and to ground GND. The PNP transistor 150 is OFF due to the first polarity of the input signal, and hence, is “seen” as high impedance “open circuit.” The voltage across the resistor 152 also biases the second PNP transistor 156 in the power stage 144, placing a first polarity voltage (+VBE) across the base-emitter junction of transistor 156. The biasing of the second PNP transistor 156 allows current to pass through the emitter-collector junction, which current is an amplified replica of the first half of the input signal, and is fed to the load 158. During the first half cycle of the input signal, the first PNP transistor 150 and the second NPN transistor 162 are OFF. Both transistors 150 and 162 function as “open circuit” and are “seen” as high impedance elements during this first half cycle of the input signal.
It should be noted that there are no current sources with class B amplifiers, and hence, before the second half cycle of the input signal commences (during the zero-crossing of the input signal), all of the transistors are turned OFF due to the fact that the input signal strength is near zero, which is below the biasing threshold of the transistors. The small glitch 160 at the link between the two halves of the input signal is therefore due to the fact that all transistors are OFF during this crossover period. Upon crossover of the input signal to a second polarity, passing the biasing threshold of either of the transistors 148 and 150, in the second half of the cycle of the input signal, the transistor 150 is activated (−VBE functions as a sink) and transistor 148 remains OFF.
Activation of the transistor 150 places a second polarity voltage across the resistor 166 that is coupled in between the negative voltage source −VCC 164 and the collector of the PNP BJT transistor 150. The current generated due to the voltage across the resistor 166 passes through the emitter-collector junction of the PNP BJT transistor 150 and to ground GND. The NPN transistor 148 is OFF due to the second polarity of the input signal, and hence, is “seen” as high impedance “open circuit.” The voltage across the resistor 166 also biases the second PNP transistor 162 in the power stage 144, placing a second polarity voltage (−VBE) across the base-emitter junction of transistor 162. The biasing of the second NPN transistor 162 allows current (sink current) to pass through the collector-emitter junction, which current is an amplified replica of the second half of the input signal, and is fed to the load 158. During the second half cycle of the input signal, the first NPN transistor 148 and the second PNP transistor 156 remain OFF. Both transistors 148 and 156 function as “open circuit” and are “seen” as high impedance elements during this second half cycle of the input signal. Accordingly, not constantly biasing ON all of transistors of a class B amplifiers during the full (or whole) cycle of the input signal produces the glitch 160 when the input signal falls below the biasing threshold of the transistors during its zero crossing.
Class D amplifiers are switching power amplifiers where all power devices are operated in ON/OFF mode. The switching elements of class D amplifier are either cut off or in saturation most of the time, allowing for high efficiencies. The high efficiency translates into reduced heat sinking, smaller size, and lighter weight. Further, in general, class D amplifiers do not suffer from crossover distortion within the audio bandwidth.
The prior art FIG. 1C is an exemplary illustration of a typical Half-Bridge class D switching power amplifier 170, with a triangle wave generator 172 creating a triangle waveform carrier frequency. In general, class D amplifiers convert the audio signal 176 into high-frequency pulses that switch the output in accordance with the audio input signal 176. Some class D amplifier use pulse width modulators to generate a series of conditioning pulses that vary in width with the audio signal's amplitude. The varying-width pulses switch the power-output transistors 182 and 184 at a fixed frequency. In general, the output of the class D amplifier is fed into a low-pass filter 190 that converts the pulses back into an amplified audio signal 192 that drives an audio system 194. This design approach produces an amplifier with better than 90% efficiency, but is much more complex than its linear counterpart (class A or class B amplifiers).
As illustrated, the basic circuit layout of the class D amplifier is substantially similar to that of linear amplifiers, such as classes A and B, with a major difference being in the signals provided to an output stage. Rather than feeding an audio waveform directly to the output stage, as is done in linear amplifiers, the class D amplifier first feeds the audio waveform into a Pulse Width Modulator (PWM) circuit that feeds modulated pulses to the output stage. By quickly switching the output stage completely ON and completely OFF with varying pulse widths, the class D amplifier is able to recreate waveforms of almost any shape, and, by filtering the switching output, sound is produced by a loudspeaker connected thereto.
FIG. 1D is a schematic illustration of a typical triangular wave generator used in prior art. It should be noted that the prior art triangular wave generators 172 illustrated in FIG. 1D require at least a two stage IC operational amplifiers to generate the triangular wave at the output OUT-1, with the output OUT-2 of the first stage producing a square wave. The triangle waveform at OUT-1 sets the resulting switching frequency of the switching power amplifier of FIG. 1C.
Referring back to FIG. 1C, conventional voltage comparator 174 compares the triangle waveform with the command input signal (audio input) at 176. The PWM signal 178 from the comparator 174 is then sent to a field effect transistor (FET) driver integrated circuit 180 that drives the output FET's 182, 184. The upper N-channel output FET 182 switches a bus voltage supplied from +VDD and the lower P-channel output FET 184 switches a bus voltage supplied from −VDD to produce a high-voltage PWM waveform 186 illustrated at test point 188. This means that the output will not be a linearly amplified replica of the input, and therefore, must be processed further. The further processing is comprised of an LC filter 190, which reproduces the audio signal (amplified) 192 at the load 194.
As indicated above, class D amplifiers yield higher efficiency than other class amplifiers through use of saturated mutually exclusive source and sink switching devices. Despite their efficiency, most class D amplifiers are comprised of complex proprietary Integrated Circuits (ICs) for control of the switches, cannot accurately reproduce the input waveform except for low fidelity applications, and are subject to power supply perturbations.
In summary, class A amplifiers produce a linearly amplified replica of an input signal, but are inefficient in terms of power usage. The push-pull class B amplifiers provide excellent efficiency (compared to class A amplifiers), but introduce crossover distortion. Class D amplifiers are efficient, and produce a fairly accurate linearly amplified replica of an input signal, but are comprised of complex proprietary Integrated Circuits (ICs) for control of the power output switches, and require power regulation for proper operation.
Accordingly, in light of the current state of the art and the drawbacks to current amplifier devices mentioned above, a need exists for an amplifier that would have simple, non-proprietary circuit topography that would allow for the use of off-the-shelf components, that would continue to be highly efficient, that would not require power supply regulations, and that would produce a linear amplified replica of an input signal.